My first inclinations was to dismiss chemical weapons as "not that bad" myself, but then I did a little checking into the sarin attack in Tokyo in 1995. While the number of deaths wasn't terribly high, twelve or so, several thousand people wound up in the hospital.
The Terrorist Attack with Sarin in Tokyo
Further, there seems to be some long term effects of sarin on memory that are concerning, though more study is needed.
Effects of Sarin on the Nervous System...
There also are a number of studies indicating that low levels of sarin exposure may be a contributing factor to Gulf War syndrome.
Gulf War Syndrome Research...
All in all, pretty nasty stuff, and while not nukes, I believe they would be more effective in a terror role than conventional explosives. As far as things like cluster bombs are concerned, it is highly unlikely that terrorists are going to have the means to deliver cluster bombs, fuel-air explosives or other "conventional" munitions. The reason we didn't want Saddam to have WMD was primarily the fear that he would pass them on to terrorists.